Microscopic machines, structures, devices, and integrated circuits (hereafter collectively called "microstructures" for simplicity) have wide application. Integrated circuits are used in devices too numerous to be recited. Microstructures other than integrated circuits, whose dimensions are typically on the order of several hundred microns down to one micron, or even into the submicron range, also have a wide range of applications. They have been used in micromechanics, microoptics, integrated optics, sensors, actuators, and chemical engineering. Microstructures that have been built include such structures as gears, nozzles, chromatographic columns, acceleration sensors, microturbines, micromotors, and linear actuators.
Microstructures are usually manufactured through a lithography process. In lithography, one or more "masks" are initially prepared, each mask incorporating all or part of the pattern to be formed on a sample surface. Transparent and opaque areas of the mask represent the desired pattern. Radiation, such as visible light, ultraviolet light, x-rays, an electron beam, or an ion beam, is transmitted through the mask onto a resist. After exposure, the resist (which may have either a positive tone or a negative tone) is developed to form the pattern on the sample surface.
To support the opaque portions of the pattern in the mask, a substrate or carrier is used that is reasonably transparent to radiation at the wavelength used for the exposure. For lithography in visible or ultraviolet wavelengths, glass has typically been used as the carrier. For x-ray masks, carriers have typically been expensive membranes a few microns thick, usually made of a low-Z ("Z"=atomic number) material such as silicon, beryllium, titanium, aluminum, silicon nitride, or graphite.
In making an integrated circuit, it is usually desirable to have the "depth" of a feature (i.e., the dimension in the direction normal to the surface of the pattern) be relatively small. By contrast, in making microstructures other than integrated circuits, it is often desirable to have the "depth" of the feature be relatively large (i.e., deep-etch lithography), to impart a three-dimensional structure to the microstructure, or a reasonable degree of strength to the microstructure, or both.
The resolution of a microstructure is the dimension, in a direction parallel to the structure's surface, of the smallest reproducible feature, or the smallest reproducible gap between adjacent features. The "aspect ratio" of a microstructure is the ratio of the depth of a feature to the resolution. To the inventors' knowledge, an aspect ratio of about 50 is the highest aspect ratio that has previously actually been achieved for any microstructure having a resolution of 10 microns or smaller.
Three methods of imaging have previously been used in lithography. In proximity imaging, the mask is positioned a small but finite distance (or gap) from the sample surface. Proximity imaging is predominantly used in x-ray lithography.
In projection imaging, a projection lens is placed between the mask and the sample to focus light onto the sample surface. (Alternatively, a condenser lens may be placed before the mask.) Projection imaging is predominantly used in visible and ultraviolet optical lithography.
In contact (or zero-gap) printing, the mask is placed directly on (but not adhered to) the sample surface. Contact printing is rarely used in lithography. In x-ray lithography, contact printing can result in serious contamination problems for the sample, and in degraded integrity for the mask. In optical lithography, contact printing can result in deterioration of the image from uncontrolled multiple reflections and light interferences between the mask and the sample surface.
Producing masks for lithographic applications can be expensive. X-ray masks are particularly expensive, typically costing about three to five times as much as an optical mask. The cost of mask production has two principal components: the cost of the substrate, and the cost of pattern formation. For a master mask having a moderate-density pattern, these two cost components are often roughly equal. But for a high-density pattern, such as is typical of a mask for an integrated circuit, the cost to produce the pattern can be many times higher than the cost of the substrate. On the other hand, for masks used in x-ray micromachining, the cost of the substrate is typically higher. In the latter case, the substrate provides the rigidity and integrity needed by the mask, which is subjected to the heavy impact of high doses of ionizing radiation.
"LIGA--Movable Microstructures," Kernforschungszentrum Karslruhe (1993?) reported the manufacture of an acceleration sensor having a height of 100 microns, a cantilever 10 microns wide, and slit width of 4 microns, i.e., an aspect ratio of 25. See the left-hand column of page 8 of that publication.
J. Mohr et al., "Herstellung von beweglichen Mikrostrukturen mit dem LIGA-Verfahren," KfK Nachrichten, Jahrgang 23, 2-3/91, pp. 110-117 (1991) reported the manufacture of a structure having a gap width of 3 microns, and a depth of 150 microns, i.e., an aspect ratio of 50. See FIG. 6 of Mohr et al.
E. Spiller et al., "X-Ray Lithography," Research Report, I.B.M. T. J. Watson Research Center (1977) disclosed a technique for x-ray microscopy of biological objects in which a specimen is brought in close contact with the resist surface, either mounted on a thin substrate, a grid, or directly on top of the resist layer. After development a relief structure is obtained in which the heights of a feature in the resist correspond to the x-ray absorption of the specimen. A scanning electron microscope was used to produce a magnified picture of the resist relief image. See pp. 48-49 and FIGS. 3.28 and 3.29 of Spiller et al.